Shingled writing is a form of PMR (perpendicular magnetic recording) and has been proposed for future high density magnetic recording by R. Wood et al. in “The Feasibility of Magnetic Recording at 10 Terabits Per Square Inch on Conventional Media”, IEEE Trans. Magn., Vol. 45, pp. 917-923 (2009). In this scheme, tracks are written in a sequential manner from an inner diameter (ID) to an outer diameter (OD), from OD to ID, or from OD and ID towards a middle diameter (MD) in a radial region of a disk in a hard disk drive (HDD). In other words, a first track is partially overwritten on one side when a second track adjacent to the first track is written, and subsequently a third track is written that partially overwrites the second track, and so forth. Track widths are defined by the squeeze position or amount of overwrite on the next track rather than by the write pole width as is the case in today's hard drives.
One of the main advantages of shingled writing is that write pole width no longer needs to scale with the written track width. Thus, the opportunity for improved writability and higher device yield is not restricted by using pole width as a critical dimension to be tightly controlled. Secondly, adjacent track erasure (ATE) becomes less of an issue because tracks are written sequentially in a cross-track dimension and only experience a one time squeeze from the next track. On the other hand, shingled writing has a unique issue that must be encountered. Because of writing from only one corner of the write pole and the one side squeeze associated with each track, track edge writing and transition curvature have larger impacts on the written tracks compared to conventional writing. This effect is depicted in FIG. 1 where four tracks A-D are sequentially overlaid on one another such that track B partially overwrites track A, track C partially overwrites track B, and track D overwrites a side of track C. Due to the one-side squeeze and the shape of the field contour at one side of the write pole, written transitions will be skewed even when the writer and reader are at zero skew. Note that the transitions shown as alternating light and dark colored bands are all sloped (skewed) with respect to the cross-track direction for tracks A-C that have a one side squeeze.
Referring to FIG. 2, micromagnetic simulated written tracks also confirm that transitions in one-sided squeezed tracks are skewed. The simulated track image is taken from a paper DC-08 entitled “Write head modeling for shingled writing” by Y. Kanai during a presentation at Intermag 2009 in Sacramento, Calif. A series of tracks is written from right to left with zero degree writer skew. The track depicted within rectangle 12 was squeezed from the left side and clearly has transitions tilted in one direction as indicated by dashed line 14. Transitions 10 generally alternate with transitions 11 in a down track direction. The last written track shown within rectangle 13 is representative of written tracks in current hard drives and has a left-right symmetric transition curvature as indicated by dashed lines 15. A hypothetical reader 12r is overlaid on the squeezed track within rectangle 12 to illustrate the impact of the shingled written track. In this example, there is an angle of 10 degrees between the transition slope and reader 12r which will cause read back pulse width and resolution degradation thereby affecting SNR and bit error rate (BER) performance. On the other hand, a hypothetical reader 13r overlaid on the center of the last written track in rectangle 13 indicates there is no effect of the transition curvature 15 on SNR and BER in conventional tracks. However, a conventional writer pole design cannot meet the requirements of shingled writing. Thus, an improved write pole structure is needed to reduce the transition curvature and skew of written tracks in shingled writing at zero head skew.
Although it may appear trivial to change the skew profile of the head across the stroke to compensate for transition skew in tracks that are squeezed on one side during a shingled writing process, it should be understood that such an adjustment will not solve the aforementioned issue of relative skew between the written tracks and the reader since the reader and writer skews both change together when done on a slider at head gimbal assembly (HGA) level. Therefore, a solution is still needed to compensate for the relative skew between reader and written tracks in shingled writing.
A search of the prior art revealed the following references. In U.S. Pat. No. 6,967,810, a shingled writing method is described in which a random update zone where the write element has no skew can be interposed between inner and outer regions of a disk. In related U.S. Pat. No. 7,490,212, a shingled writing method is disclosed wherein two or more contiguous data tracks establish a band, and a band may store data from only one file. U.S. Pat. No. 7,372,656 describes a shingled writing direction that is defined by assigning Logical Block Addresses (LBAs) from an outer circular side to an inner circular side for both surfaces of a recording medium.